Log into http://calendar.live.com/. Click the Options gear, then Options. In the Options pane, under the Calendar section, select "Publish calendar". The ICS URL will be displayed.
Right-click and choose Save As to save it to your hard drive.

Thanks for that Brian.
However, the HTML and ICS links do not appear. All that appears are: 'select a calendar, show availability only / create, and show availability titles and locations / create'. When either of the Create options are clicked the same error is always displayed
( from two separate Win 10 pcs and from an Ubuntu device ):-
'The user record already exists in Store. Possible reason - current session has MbxReadMode flag set to NoMbxRead. You need to read existing user from Store first before you can modify it.'
I do not know what this means. It looks like a reference to the Windows pc registry, but since Find hangs in Win 10 regedit and regedt32 I don't know if it is there. In any case if it did refer to the local registry why would it appear when accessing Outlook.com
from Ubuntu.